Network details

Our system is divided in three modules:

The signaling module

The signaling module allows our bacteria strain to integrate the input signal = the presence of a pathogene.

You can find here the mathematical model and numerical simulation this module.

The idea behind this module comes from the iGEM London Imperial College 2010 Team's work on Parasight [1].

Staphylococcus aureus secretes the exfoliative toxin B [2] which cleaves a specific amino-acids sequence (Desmoglein 1). This specific sequence can be used as a linker between a membrane protein and a dipeptide.
Once S. aureus is present, the linker is cut by the toxin and the dipeptide is released.

The dipeptide binds an engineered receptor [3] [4] that consists of:

the extracellular part of Tap [5], a dipeptide receptor involved in the chemotaxism
the intracellular part of EnvZ [6], a histidine kinase involved in the osmoregulation

Once the dipeptide binds the Tap part [7], the intracellular EnvZ part allows the phosphorylation of OmpR [8] [9], which is a constitutively produced transcriptional activator.

OmpR phosphorylation allows the activation of the ompC promoter [10]. We introduced cyaA (that code for adenyl cyclase) downstream of this promoter.

Amplification module

The amplification module allows our bacteria to amplify the input signal and to produce an output signal = fluorescence.

As for the previous module you can read here our mathematical model and numerical simulation.

The activation of the ompC promoter allows the production of Adenyl cyclase [11]. Adenyl cyclase catalyses the conversion of ATP (Adenosine Tri-Phosphate) into cAMP (cyclic Adenosine Mono-Phosphate).

The binding of cAMP to CRP (cAMP Receptor Protein) leads to the production of AraC by activating the pmalT promoter [12].
In the presence of arabinose, AraC and cAMP-CRP, cooperatively activate the paraBAD promoter [13], thus forming an "AND" gate. This allows the production of:

Adenyl cyclase which reproduces cAMP, forming thus a positive amplification loop
GFP (Green Fluorescent Protein) = the output signal

Cell to cell communication module

In a recent study a new role of cAMP was described (A synthetic Escherichia coli communication system mediated by extracellular cyclic AMP (publication in progress)). This system is involved in bacterial communication. We used this module to allow communications within our bacteria population
As for the previous module you can read here our mathematical model and numerical simulation.

When a bacterium detects S. aureus, it produces several molecules of GFP and even more cAMP. cAMP diffuses through the membrane and activates the amplification loop in neighboring bacteria [14], which triggers in turn the production of GFP and cAMP.
This leads to GFP production by the entire population, triggered by a single bacterium that has detected the pathogen in the first place: